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sql server update的赋值次序_数据库技巧

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1) 先变量再字段


SET NOCOUNT ON;


DECLARE @i INT, @j INT
DECLARE @Table TABLE
(
 Id1 INT,
 Id2 INT
);


INSERT @Table(Id1, Id2)
SELECT 1, 10
UNION ALL
SELECT 2, 20
UNION ALL
SELECT 3, 30;


SELECT @i = 1, @j = 0
UPDATE @Table SET Id1 = @i, Id2 = Id1, @i = @i + 1
SELECT * FROM @Table


SELECT @i = 1, @j = 0
UPDATE @Table SET Id1 = @i, Id2 = @j, @j = @i + 10, @i = @i + 1
SELECT * FROM @Table


SET NOCOUNT OFF;
结果:
Id1         Id2        
———– ———–
2           1
3           2
4           3


Id1         Id2        
———– ———–
2           11
3           12
4           13



2) 变量之间, 从左到右


SET NOCOUNT ON;


DECLARE @i INT, @j INT
DECLARE @Table TABLE
(
    Id1 INT,
    Id2 INT
);


INSERT @Table(Id1, Id2)
SELECT 1, 10;


SELECT @i = 1, @j = 0
UPDATE @Table SET @j = @i, @i = @i + 1
PRINT @i = + CAST(@i AS VARCHAR) + , @j = + CAST(@j AS VARCHAR)


SELECT @i = 1, @j = 0
UPDATE @Table SET @i = @i + 1, @j = @i
PRINT @i = + CAST(@i AS VARCHAR) + , @j = + CAST(@j AS VARCHAR)


SELECT @i = 1, @j = 0
UPDATE @Table SET @i = @j + 1, @j = @i
PRINT @i = + CAST(@i AS VARCHAR) + , @j = + CAST(@j AS VARCHAR)


SELECT @i = 1, @j = 0
UPDATE @Table SET @i = @j, @j = @i
PRINT @i = + CAST(@i AS VARCHAR) + , @j = + CAST(@j AS VARCHAR)


SET NOCOUNT OFF;
结果:
@i = 2, @j = 1
@i = 2, @j = 2
@i = 1, @j = 1
@i = 0, @j = 0



3) 字段之间, 并行执行


SET NOCOUNT ON;


DECLARE @Table TABLE
(
 Id1 INT, Id2 INT
);


INSERT @Table(Id1, Id2)
SELECT 1, 10
UNION ALL
SELECT 2, 20
UNION ALL
SELECT 3, 30;


UPDATE @Table SET Id1 = Id2, Id2 = Id1
SELECT * FROM @Table


SET NOCOUNT OFF;
结果:
Id1         Id2        
———– ———–
10          1
20          2
30          3

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